Abstract

Endoplasmic reticulum (ER) stress causes pancreatic β-cell dysfunction and contributes to β-cell loss and the progression of type 2 diabetes1,2. Wolfram syndrome 1 (WFS1) has been shown to be an important regulator of the ER stress signalling pathway3; however, its role in β-cell function remains unclear. Here we provide evidence that WFS1 is essential for glucose- and glucagon-like peptide 1 (GLP-1)-stimulated cyclic AMP production and regulation of insulin biosynthesis and secretion. Stimulation with glucose causes WFS1 translocation from the ER to the plasma membrane, where it forms a complex with adenylyl cyclase 8 (AC8), an essential cAMP-generating enzyme in the β-cell that integrates glucose and GLP-1 signalling4. ER stress and mutant WFS1 inhibit complex formation and activation of AC8, reducing cAMP synthesis and insulin secretion. These findings reveal that an ER-stress-related protein has a distinct role outside the ER regulating both insulin biosynthesis and secretion. The reduction of WFS1 protein on the plasma membrane during ER stress is a contributing factor for β-cell dysfunction and progression of type 2 diabetes.

Recent studies reveal that ER stress probably contributes to pancreatic β-cell dysfunction and may be the tipping point for the β-cell loss seen in patients with type 2 diabetes1,2,5. WFS1 is an ER membrane glycoprotein highly expressed in the insulin-producing β-cells of islets, and regulates the ER stress signalling pathway3,6,7. Both humans and mice deficient in WFS1 have β-cell loss, possibly as a result of a dysregulated ER stress response8,9. Therefore, WFS1 is important for ER homeostasis in the β-cell; however, its role in β-cell function is unknown.

WFS1 is required for insulin production, storage and secretion in β-cells. (a) Relative expression of Ins1 and Ins2 measured by quantitative PCR (Ins2 represented in figure; similar for Ins1; n = 3) in rat islets transduced with either GFP or...

Over 100 mutations in WFS1 have been identified in patients with Wolfram syndrome, a disease characterized by juvenile diabetes and optical atrophy10. We overexpressed three WFS1 mutants in an INS-1 832/13 β-cell line in which endogenous WFS1 protein was suppressed using inducible short hairpin RNA (shRNA) against Wfs1. Insulin gene expression in these cells was reduced by 43–45% (P < 0.01, n = 3; Fig. 1i). Figure 1j shows that GSIS in the mutant cells was suppressed by 45–52% (P < 0.01, n = 3). A similar suppression of insulin gene expression and GSIS was observed in INS1 832/13 cells with >90% knockdown of WFS1 in the absence of mutant WFS1 expression (Fig. 1i,j). Insulin gene expression and GSIS were similar to control in cells co-expressing endogenous and mutant WFS1 (Fig. 1i,j), and was restored in knockdown cells with re-expression of wild-type WFS1 (data not shown). Consistent with our data in cell lines expressing mutant WFS1 and a previous report11, we found reduced insulin gene expression (55 ± 5%, P < 0.05, n = 3; Fig. 1k), insulin content (47 ± 4%, P < 0.01, n = 3; Fig. 1l) and GSIS (40 ± 9%, P < 0.05, n = 3; Fig. 1m) in islets isolated from WFS1-deficient (Wfs1−/−) mice when compared with islets obtained from wild-type littermates. The defect in GSIS in Wfs1−/− islets can be rescued with WFS1 re-expression (Fig. 1n). We next investigated whether WFS1 could modulate insulin processing by measuring the secreted proinsulin to insulin ratio. In rat islets overexpressing WFS1 and in Wfs1−/− islets, this ratio is comparable to control islets (Supplementary Fig. S1a,b). Together, these data demonstrate that WFS1 is required for both insulin biosynthesis and GSIS.

Next we investigated the possibility that WFS1 modulates insulin production and release through cAMP, a second messenger regulating both insulin biosynthesis and secretion12–14. Figure 2a shows that overexpression of WFS1 in rat islets increases glucose-induced cellular cAMP synthesis by 90 ± 11% (P < 0.01, n = 3). Consistent with the idea that enhancement of insulin production and release by cAMP is mediated in part by protein kinase A (PKA) activation15, we found that the PKA inhibitor H-89 suppressed insulin gene expression (Fig. 2b) and GSIS in WFS1-overexpressing islets (Fig. 2c). Glucose stimulation of INS-1 832/13 cells expressing three WFS1 mutants did not enhance cAMP production and was reduced by 78 ± 13% (P < 0.01, n = 3; Fig. 2d). Similar data were observed in INS-1 832/13 with >90% knockdown of WFS1 (Fig. 2d). cAMP responsiveness to glucose stimulation was unaffected in cells expressing both endogenous and mutant WFS1 (Fig. 2d) and was restored in mutant cell lines and Wfs1−/− islets with re-expression of wild-type WFS1 (data not shown). Cells with WFS1 knockdown also had a small but significant defect in the amount (Fig. 2e) and rate (Fig. 2f) of glucose-induced Ca2+ increase ([Ca2+]i). This defect can be explained, in part, by the suppression of key β-cell genes that comprise the glucose-sensing machinery (Supplementary Fig. S2). These data demonstrate that the main effect of WFS1 on insulin release is distal of [Ca2+]i.

WFS1 is an important regulator of the ER stress pathway7. Therefore, we explored the effect of ER stress on WFS1-mediated and glucose-stimulated cAMP production and insulin secretion. Although the defect in cAMP production and insulin release is apparent in 5-week-old Wfs1−/− islets and progresses with age (Fig. 3a,b), ER stress, defined by increases in the following ER stress markers: BiP, CHOP, activating transcription factor 4 (ATF4) and sXBP1 (Fig. 3c–f), is not noteworthy in these 5-week-old islets. We then assessed whether mitigation of ER stress in older Wfs1−/− islets with the chemical chaperone 4-phenylbuytric acid (4-PBA) could rescue both the cAMP and GSIS defect (Fig. 3g–l). 4-PBA significantly reduced ER stress in Wfs1−/− islets (Fig. 3i–l); however, this did not translate into a restoration of β-cell function (Fig. 3g,h). This suggests that it is the absence of WFS1 expression and not general ER stress that causes the β-cell functional defects in Wfs1−/− islets. We also found that treatment of control islets with the ER stress inducer thapsigargin (Tg) caused a reduction in cAMP production and GSIS (Fig. 3m,n). Overexpression of WFS1, however, enhanced both cAMP synthesis (95 ± 6%, P < 0.01, n = 3; Fig. 3m) and GSIS (36 ± 4%, P < 0.05, n = 3; Fig. 3n) in the presence of Tg when compared with untreated control cells. These data suggest that WFS1 is critical for β-cell function and overexpression of WFS1 can restore cAMP signalling and insulin release during ER stress insult.

GLP-1 and glucose-dependent insulinotropic hormone (GIP) are important gut hormones that stimulate insulin release through enhancement of cAMP signalling16. As expected, GLP-1 and GIP increased cAMP levels (Fig. 4a) and insulin secretion in wild-type mouse islets (Fig. 4b). Interestingly, glucose along with GLP-1 and GIP lacked the ability to increase cAMP and insulin release in Wfs1−/− islets (Fig. 4a,b). These islets also had an impaired response to forskolin (Fig. 4a,b), a diterpene shown to enhance cAMP synthesis17. The lack of an insulin secretory response in the Wfs1−/− islets is due to the lack of cAMP production and not a defect in the insulin secretory machinery, as the membrane-permeable cAMP analogue, dibutyl cAMP (DBcAMP), stimulated insulin release in Wfs1−/− islets similar to that observed in islets isolated from wild-type littermates (Fig. 4b). This defect also cannot be attributed to upregulation of cAMP hydrolysis by phosphodiesterases (PDEs), as treatment of Wfs1−/− islets with the PDE inhibitor, 3-isobutyl-1-1methylxanthine (IBMX) did not enhance cAMP accumulation or insulin secretion (Fig. 4a,b). GLP-1 and GIP receptor expression was unchanged in Wfs1−/− islets (data not shown). Furthermore, the sulphonylurea glyburide and potassium chloride (KCl) produced robust and similar increases in insulin secretion in wild-type and Wfs1−/− islets (Fig. 4b). Wfs1−/− islets also responded similarly to wild-type islets when treated with the protein kinase C activator, phorbol 12-myristate 13-acetate (data not shown). These data demonstrate that WFS1 regulates glucose- and gut-hormone-induced cAMP production and insulin release in the β-cell, but does not affect the ability of cAMP-independent pathways to stimulate insulin secretion.

cAMP production is regulated through the activation of multiple adenylyl cyclase isoforms15. We confirmed that WFS1 enhancement of cAMP is secondary to upregulation of total adenylyl cyclase activity (Fig. 4c). Glucose and GLP-1 have been shown to specifically activate the Ca2+/calmodulin-dependent AC8 isoform4,18,19, suggesting that WFS1 regulation of cAMP may be through regulation of AC8. We further investigated whether WFS1-mediated cAMP production is due to modulation of AC8 expression or activity. AC8 gene and protein expression were unchanged by WFS1 overexpression in rat islets and INS-1 832/13 cells, as well as in Wfs1−/− islets (Supplementary Fig. S3a–e). To probe for a possible interaction between the WFS1 and AC8 proteins, endogenous AC8 was immunoprecipitated from INS-1 832/13 lysates with or without overexpression of WFS1 (Fig. 4d,e). Western blot analyses provided evidence of complex formation between WFS1 and AC8 (Fig. 4d, lanes 9–12), as well as between WFS1 and calmodulin (Supplementary Fig. S4a, lanes 5 and 6). Stimulatory glucose increased endogenous WFS1 complex formation with AC8 (85 ± 9%, P < 0.01, n = 3; Fig. 4e) and with calmodulin (320 ± 28%, P < 0.01, n = 3; Supplementary Fig. S4b). Overexpression of WFS1 enhanced complex formation with AC8 (111 ± 10%, P < 0.01, n = 3) and between AC8 and calmodulin (101 ± 13%, P < 0.05, n = 3; Fig. 4e). No complex formation was seen between WFS1 and the other Ca2+/calmodulin-dependent adenylyl cyclase isoform, AC1 (Fig. 4f,g), nor with AC6 (Supplementary Fig. S5a,b), confirming the specificity of the WFS1–AC8–calmodulin complex. As expected, WFS1 did not complex with other cAMP pathway proteins such as protein phosphatase 2 (PP2A and Supplementary Fig. S5c), PKA (Supplementary Fig. S5d) and exchange protein directly activated by cAMP 2 (Epac2; Supplementary Fig. S5e), and important PDEs of the β-cell, PDE3b and PDE7 (ref. 20 and Supplementary Fig. S5f,g). Moreover, in INS-1 832/13 cells expressing WFS1 mutations, the glucose-dependent WFS1–AC8–calmodulin complex formation was not evident (Fig. 4h, lanes 5 and 6) and, more importantly, the AC8–calmodulin complex was also not detected (Fig. 4h,i).

We further explored the formation of the WFS1–AC8–calmodulin complex during ER stress. Consistent with our data showing impairment of cAMP signalling with ER stress (Fig. 3m), we found that Tg treatment prevented WFS1–AC8–calmodulin complex formation (Fig. 4j, lane 8). WFS1–AC8 complex formation in control cells was reduced (71 ± 15%, P < 0.01, n = 3) and AC8–calmodulin complex formation was also reduced (48 ± 9%, P < 0.01, n = 3) with Tg treatment (Fig. 4k). WFS1–AC8 and AC8–calmodulin complex formation with Tg treatment remained significantly higher in cells overexpressing WFS1 than in control cells (Fig. 4k). Confirming previous studies18, knockdown of AC8 in INS-1 832/13 cells by >60% suppressed GSIS by 48 ± 8% (P < 0.05, n = 3; Fig. 4l). This defect could not be rescued by WFS1 overexpression (Fig. 4l). In accordance, the enhancement of GSIS in rat islets by WFS1 overexpression was abolished by treatment with a general adenylyl cyclase inhibitor (data not shown). These data suggest that WFS1 is an essential component in activated AC8–calmodulin complex formation, a step required for β-cell cAMP synthesis, and this complex can be disrupted by ER stress causing β-cell dysfunction.

WFS1 was identified as an ER transmembrane protein7; thus, complex formation with the plasma membrane protein AC8 needed further investigation. We assessed whether WFS1 could translocate out of the ER following glucose stimulation to form a complex with AC8 and calmodulin. WFS1 does not contain traditional ER retention sequences6; therefore, we investigated whether it could be held in the ER under steady-state conditions by an ER-resident chaperone, similar to the way other ER membrane proteins are retained21. Previous studies indicate that WFS1 and the ER chaperone glucose-regulated protein 94 (GRP94) form a complex in neuronal cells22. Here, we show that WFS1 and GRP94 form a glucose-dependent complex in β-cells. When GRP94 is immunoprecipitated from INS-1 832/13 cells, a strong interaction is seen under basal conditions (Fig. 5a, lane 6), which weakens with glucose stimulation (Fig. 5a, lane 7 and Fig. 5b), but reforms in the presence of Tg treatment (Fig. 5a, lane 8 and Fig. 5b). These results suggest that under basal conditions, GRP94 retains WFS1 in the ER and when stimulated with glucose, GRP94 releases WFS1 allowing WFS1 to escape the ER. In the presence of ER stress, the GRP94–WFS1 complex reforms, presumably to maintain WFS1 in the ER to fulfil its function in ameliorating ER stress; however, this leads to an overall reduction in β-cell function as seen in Fig. 3m–n.

WFS1 translocates from the ER to the plasma membrane in a glucose-dependent manner. (a) GRP94 was immunoprecipitated (IP) from cell lysates prepared from INS-1 832/13 cells treated for 2 h with either 2.5 mM glucose (2.5G) or 16.7 mM glucose (16.7G),...

To determine whether WFS1 forms a complex with AC8 and calmodulin at the plasma membrane, we performed plasma membrane fractionation studies (Fig. 5c). To confirm enrichment of the plasma membrane, the fractionation samples were probed with anti-Na+/K+ ATPase (Fig. 5c). Both AC8 and calmodulin were present at the plasma membrane under basal and stimulated conditions (Fig. 5c, lanes 5 and 7), whereas GRP94 and another ER luminal protein, BiP, were excluded from these fractions (Fig. 5c, lanes 5–8). Under basal conditions, 20 ± 6% (P < 0.05, n = 3) of total WFS1 was found at the plasma membrane, which increased to 46 ± 3% (P < 0.01, n = 3) on stimulation with glucose but was reduced to levels similar to basal conditions with Tg treatment (Fig. 5d), suggesting the importance of ER localization of WFS1 during ER stress.

We further evaluated the translocation of WFS1 from the ER by immunocytochemistry (Fig. 5e). As expected, WFS1 co-localizes with the ER marker protein disulphide isomerase (PDI) in unstimulated cells (Fig. 5e, (1)); however, the WFS–PDI co-localization coefficient ratio is reduced (40 ± 5%, P < 0.01, n = 3) with glucose stimulation (Fig. 5e, (2) and Fig. 5f), which corresponds to an increase in the co-localization coefficient between WFS1 and the Golgi marker giantin (Fig. 5e, (4) and Fig. 5f). Consistent with a recent study23, WFS1 co-localizes with a marker of insulin granules, chromagranin A (ChrA; Fig. 5e, (5, 6)), which also increases (90 ± 11%, P < 0.05, n = 3) with glucose stimulation (Fig. 5f). In line with our plasma membrane fraction data, WFS1 co-localizes with the plasma membrane marker β-catenin (bCat) in both unstimulated and stimulated cells (Fig. 5e, (7, 8)). Collectively, these findings suggest that glucose causes a translocation of WFS1 from the ER to the Golgi and stimulates the accumulation of WFS1 on the plasma membrane, where it forms a complex with AC8–calmodulin, and stimulates insulin biosynthesis and secretion.

Together, these data provide a more detailed understanding of insulin biosynthesis and secretion in the pancreatic β-cell, and provide insight into a mechanism by which β-cells balance ER stress with overall function. Our results demonstrate that WFS1 is essential for the active complex formation of AC8 and calmodulin, which is required for both cAMP synthesis and amplification of insulin expression and secretion, and that this process is perturbed by ER stress (Fig. 5g). ER stress traps WFS1 in the ER membrane, preventing AC8–CaM complex formation and activation of the cAMP signalling cascade. Although it is possible that ER stress may not be exclusively affecting WFS1 shuttling to the plasma membrane, we propose that its role in regulation of WFS1 translocation is a negative feedback loop to reduce ER workload during stress by preventing insulin biosynthesis. This is the first example of the requirement of an ER-resident protein for glucose and gut hormone signalling in the β-cell. Previous reports demonstrate a clear relationship between defects in cAMP signalling and diabetes24,25, as well as between ER stress and diabetes1,2. On the basis of our data, WFS1 is a key link between ER stress and β-cell function. Although it is clear that WFS1 has dual roles in ER stress regulation and cAMP generation, it remains unclear whether these functions are entirely independent. We therefore postulate that in patients with type 2 diabetes, pathological ER stress contributes to β-cell dysfunction through a defect in the translocation of WFS1 to the plasma membrane.

METHODS

Cell culture and plasmids

For generation of β-cells with inducible overexpression of WFS1 or GFP, INS-1 832/13 cells were transduced with the tetracycline repressor, pTetR Tet-On (TO), and then transduced with a lentivirus expressing human WFS1–FLAG or GFP. For inducible overexpression, cells were cultured in 2 µM doxycycline for 48 h before secretion assays and protein/RNA isolation. For generation of cells with inducible knockdown of WFS1, INS-1 832/13 pTetR TO cells were transduced with a lentivirus expressing shRNA against rat Wfs1, and cultured in 2 µM doxycycline for 48 h before protein/RNA isolation and secretion assays. For generation of stable mutant WFS1 cell lines, INS-1 832/13 pTetR TO shWFS1 cells were transfected with mutant WFS1 plasmids using Lipofectamine 2000 (Invitrogen) and cultured in 2 µM doxycycline for 48 h to suppress endogenous WFS1 expression before protein/RNA isolation and secretion assays. Wolfram syndrome disease mutant plasmids (P724L, ins483fs/ter544, G695V) were generated as previously described3.

Primary islet isolation and transduction

Male Sprague Dawley rats weighing 250–300 g (Charles River) were anaesthetized by intraperitoneal injection with a cocktail of ketamine, xylazine and acepromazine. Pancreatic islets were isolated as previously described26. Five- and ten-week-old pancreatic β-cell-specific Wfs1-knockout and Cre-positive control mouse islets were generated as previously described9. All animal procedures were performed in accordance with animal welfare regulations under an approved institutional Animal Care and Use Committee protocol in the Novartis Institute for Biomedical Research and University of Massachusetts Medical School animal facilities, which are accredited by the Association for Assessment and Accreditation of Lab Animal Care. Human islets were obtained from Prodo Laboratories (www.prodolabs.com). Islets were plated on laminin-5 (Chemicon)-coated tissue culture dishes 18 h post-isolation, and transduced with 1.0 × 10(ref. 9) viral particles per millilitre of lentivirus expressing human WFS1 or GFP. At 16 h post-transduction, the medium was changed and the islets were incubated for 24 h before performing secretion assays and collecting RNA/protein.

Glucose-stimulated secretion assays

Cell lines and primary rodent islets were pre-incubated with 5 mM glucose overnight, and then stimulated with 2.5 mM or 16.7 mM glucose for 2 h. Proinsulin secretion, insulin secretion and insulin content were measured by the Luminex assay, cellular cAMP was measured by ELISA according to the manufacturer's protocol (Cell Biolabs) and adenylate cyclase activity was measured using a cAMP HiRange membrane assay HTRF according to the manufacturer's protocol (Cisbio). Insulin secretion was normalized by DNA content using CyQUANT (Invitrogen) or insulin content as described above. DBcAMP, IBMX, KCl and 4-PBA were purchased from Sigma; GIP, GLP-1 and forskolin were purchased from American Peptide Company; glyburide and H-89 were purchased from Calbiochem.

Immunoprecipitation

Cells were lysed as previously described7. For immunoprecipitation of endogenous AC8, CaM or GRP94, 1 mg whole-cell extract from each sample was incubated with 50 µl Protein A/G agarose (Sigma) and 4 µg anti-AC8, anti-CaM, anti-AC1, anti-AC6, anti-GRP94, anti-PP2A, anti-PKA, anti-Epac2, anti-PDE3b or anti-PDE7 antibody overnight at 4 °C with rotation. After incubation, the agarose was washed four times with TNE buffer (50 mM Tris–HCl, 150 mM NaCl, 1 mM EDTA and 1% NP-40) followed by a final wash in 1× PBS (pH 7.4). The immunoprecipitation products were resolved by SDS–PAGE and then immunoblotted. As a control, the lysates were subject to immunoprecipitation as described above using rabbit or mouse IgG.

Real-time polymerase chain reaction

Total RNA was isolated from cells using the RNeasy Mini Kit and reverse transcribed using 1 µg of total RNA from cells with oligo-dT primer. For thermal cycling, a 7900HT Fast Real-time PCR system (Applied Biosystems) was used at 95 °C for 10 min, then 40 cycles at 95 °C for 10 s, and at 55 °C for 30 s. The relative amount for each transcript was calculated by a standard curve of cycle thresholds for serial dilutions of the complementary DNA sample and normalized to β-actin expression. The PCR was performed in triplicate for each sample; all experiments were repeated three times. The following primers were used: rat actin, 5′-GCAAATGCTTCTAGGCGGAC-3′ and 5′-AAGAAAGGGTGTAAAACGCAGC-3′; rat insulin 1, 5′-GTCCTCTGGGAGCCCAAG-3′ and 5′-ACAGAGCCTCCACCAGG-3′; rat insulin 2, 5′-ATCCTCTGGGAGCCCCGC-3′ and 5′-AGAGAGCTTCCACCAAG-3′; rat BiP, 5′-TGGGTACATTTGATCTGACTGGA-3′ and 5′-CTCAAAGGTGACTTCAATCTGGG-3′; rat Chop, 5′-AGAGTGGTCAGTGCGCAGC-3′ and 5′-CTCATTCTCCTGCTCCTTCTCC-3′; rat spliced Xbp-1, 5′-CTGAGTCCGAATCAGGTGCAG-3′ and 5′-ATCCATGGGAAGATGTTCTGG-3′; mouse actin, 5′-GCAAGTGCTTCTAGGCGGAC-3′ and 5′-AAGAAAGGGTGTAAAACGCAGC-3′; for mouse BiP, 5′-TTCAGCCAATTATCAGCAAACTCT-3′ and 5′-TTTTCTGATGTATCCTCTTCACCAGT-3′; mouse Chop, 5′-CCACCACACCTGAAAGCAGAA-3′ and 5′-AGGTGAAAGGCAGGGACTCA-3′; mouse spliced Xbp-1, 5′-CTGAGTCCGAATCAGGTGCAG-3′ and 5′-GTCCATGGGAAGATGTTCTGG-3′; mouse insulin 1, 5′-GAAGTGGAGGACCCACAAGTG-3′ and 5′-CTGAAGGTCCCCGGGGCT-3′; mouse insulin 2, 5′-TGCTGATGCCCTGGCCTGCTCT-3′ and 5′-CTGGTCCCACATATGCACATGCA-3′; mouse Glut2, 5′-GTGTGAGGATGAGCTGCCTAAA-3′ and 5′-TTCGAGTTAAGAGGGAGCGC-3′; mouse Atf4, 5′-GAAACCTCATGGGTTCTCCA-3′ and 5′-AGCTCATCTGGCATGGTTTC-3′; mouse Gck, 5′-GAAAAGATCATTGGCGGAAA-3′ and 5′-CCCAGAGTGCTCAGGATGTT-3′; mouse Sur1, 5′-ATTGCTGCCGTCCTCATTAC-3′ and 5′-CTGCACTGGACAGGAACTCA-3′; mouse Pc1/3, 5′-TTGGCTGAAAGGGAAAGAGA-3′ and 5′-GCTTCATGTGCTCTGGTTGA-3′; mouse Kir6.1, 5′-CACAAGAACATCCGAGAGCA-3′ and 5′-TCTTCTCCATGGTGCCTTTC-3′.